It optimizes the namespace declaration, exporting all contents of a same namespace within a single block.

First, it will look for TypeScript declaration namespaces:

namespace myModule {
    export declare class FromFile1 {}
}
namespace myModule {
    export declare interface FromFile2 {}
}
// ...
namespace myModule {
    export declare class FromFileN {}
}

Then, using merge it will optmize all blocks with the same module name:

namespace myModule {
    export declare class FromFile1 {}
    export declare interface FromFile2 {}
    // ...
    export declare class FromFileN {}
}